Energy consumption worldwide is generally increasing, and conventional hydrocarbon resources are being consumed. In an attempt to meet demand, the exploitation of unconventional resources may be desired. For example, highly viscous hydrocarbon resources, such as heavy oils, may be trapped in oil sands where their viscous nature does not permit conventional oil well production. Estimates are that trillions of barrels of oil reserves may be found in such oil sand formations.
In some instances these oil sand deposits are currently extracted via open-pit mining. Another approach for in situ extraction for deeper deposits is known as Steam-Assisted Gravity Drainage (SAGD). The heavy oil is immobile at reservoir temperatures and therefore the oil is typically heated to reduce its viscosity and mobilize the oil flow. In SAGD, pairs of injector and producer wells are formed to be laterally extending in the ground. Each pair of injector/producer wells includes a lower producer well and an upper injector well. The injector/producer wells are typically located in the payzone of the subterranean formation between an underburden layer and an overburden layer.
The upper injector well is used to typically inject steam, and the lower producer well collects the heated crude oil or bitumen that flows out of the formation, along with any water from the condensation of injected steam. The injected steam forms a steam chamber that expands vertically and horizontally in the formation. The heat from the steam reduces the viscosity of the heavy crude oil or bitumen which allows it to flow down into the lower producer well where it is collected and recovered. The steam and gases rise due to their lower density so that steam is not produced at the lower producer well and steam trap control is used to the same effect. Gases, such as methane, carbon dioxide, and hydrogen sulfide, for example, may tend to rise in the steam chamber and fill the void space left by the oil defining an insulating layer above the steam. Oil and water flow is by gravity driven drainage, into the lower producer well.
Various approaches are used to process the emulsion from SAGD wells. One such approach is set forth in U.S. Pat. No. 8,951,392 to James, which is directed to a modular portable evaporator system for use in SAGD systems having an evaporator, with a sump including an oil skimming weir, a short tube vertical falling film heat exchanger including an outer shell containing short tubes provided for lower water circulation rate. The system further has, external to the evaporator, a compressor for compressing evaporated steam from the tube side of the heat exchanger and routing to the shell side of the same exchanger, a distillate tank to collect hot distilled water, a recirculation pump to introduce liquids from the sump into the heat exchanger, and an external suction drum protecting the compressor from liquid impurities. The evaporator system receives produced water from the SAGD process into the sump and provides cleaned hot water to a boiler.
One consequence of the SAGD process is that it adds a significant amount of water to the emulsion output from the well, as several barrels of water (as steam) are typically injected into the well to recover one barrel of bitumen. As a result, a relatively expensive inlet diluent (e.g., naphtha) may be required, as emulsified bitumen and water have nearly the same gravity which necessitates the addition of diluents to lower the bitumen's gravity for free water knock out.
Furthermore, additional diluent may also be required during later stages of processing. In particular, most SAGD processing facilities export the extracted bitumen to refineries via pipelines. Yet, most bitumen supplies are required to include ˜30% diluent by volume to meet applicable pipeline requirements.
Accordingly, further enhancements may be desirable for bitumen extraction and treatment in certain applications.